1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image forming apparatus of the type including an image carrier made up of a conductive base and a photoconductive layer and a toner carrier to which a bias for development is applied, and transferring toner from the toner carrier to the image carrier at a developing position to thereby develop a latent image formed on the image carrier, and an image process unit for the same.
2. Description of the Background Art
An image forming apparatus of the type described includes a developing device operable with a single-ingredient type developer, i.e., toner or a two-ingredient type developer consisting of toner and magnetic grains. A developing device using a single-ingredient type developer includes a developing roller or toner carrier on which toner is directly deposited in the form of a layer. The developing roller conveys the toner to a developing position where the roller faces an image carrier carrying a toner image thereon. At the developing position, the toner layer on the toner carrier is transferred to the image carrier by an electric field, which is formed by a bias applied to the conductive base of the developing roller.
A developing device using a two-ingredient type developer includes a sleeve or toner carrier on which magnetic grains form a magnet brush. Charged toner electrostatically deposits on the magnet brush and is conveyed to a developing position in accordance with the rotation of the sleeve. At the developing position, the magnet brush with the toner adjoins or contacts an image carrier on which a latent image is formed. A bias applied to the sleeve forms an electric field in such a manner as to transfer the toner from the magnet brush to the latent image.
Japanese Patent Nos. 2,983,262 and 2,987,254, for example, each disclose a developing device operable with a single-ingredient type developer and including a developing roller or image carrier and a blade. The developing roller faces an image carrier while the blade is pressed against the developing roller in order to form a thin toner layer. More specifically, the blade charges toner deposited on the developing roller by friction while regulating the thickness of the toner layer. The thin toner layer adjoins or contacts the image carrier to thereby develop a latent image formed on the image carrier.
In the developing device described above, the blade, a toner feed roller or similar contact member controls charge to deposit on the toner by using friction. This frictional charging scheme, however, cannot readily meet a demand for high-speed charge control over toner or a demand for high durability of toner. Further, the contact member pressed against the toner carrier stresses the toner deposited on the toner carrier and is therefore apt to bring about toner filming. At the same time, the contact member is likely to cause a substance covering each toner grain to penetrate into the toner grain, deteriorating image quality. Moreover, the contact member and toner carrier wear due to friction and cause a developing characteristic to vary with the elapse of time.
The developing device using the two-ingredient type developer can deposit charged toner on the toner carrier without resorting to the blade, toner feed roller or similar contact member, i.e., friction. For example, Japanese Patent Laid-Open Publication Nos. 56-40862, 59-172662, 5-66677 and 10-240019 each propose to cause the developer to form a magnet brush on a magnet roller, magnet brush forming body or similar toner feed member. Toner contained in the magnet brush is charged to preselected polarity by friction acting between the toner and magnetic grains. Only part of the toner charged to preselected polarity is transferred from the toner feed member to the toner carrier, e.g., a developing roller or a toner layer support body.
To insure a preselected developing ability, a relatively high charge potential may be caused to deposit on the photoconductive layer of the image carrier in order to increase a potential for development, as proposed in the past. The potential for development refers to a difference between a potential deposited on the latent image of the image carrier and the bias for development. However, the relatively high potential accelerates, e.g., the electrostatic fatigue of the image carrier. In this sense, a low-voltage development using a relatively low potential is desirable. However, if the charge potential to deposit on the photoconductive layer of the image carrier is low, it is likely that the background of an image is contaminated or that the amount of toner deposition becomes too short to implement preselected image density. This problem arises without regard to the type of the developer, i.e., the single-ingredient type developer or the two-ingredient type developer.
In light of the above, we conducted a series of researches and experiments and found the following. The conductive base of the image carrier and the conductive base of the toner carrier form an equivalent circuit therebetween. By optimizing a capacity and a resistance constituting the equivalent circuit, it was possible to realize low-potential development and form images with a minimum of background contamination and with preselected density.
There is an increasing demand for an image forming apparatus featuring the sharpness of an image and the faithful reproduction of tonality. To meet this demand, a developing ability is essential that faithfully develops even latent images representative for thin lines and small dots. Further, toner must be prevented from depositing on the image carrier in an excessive amount; otherwise, the toner would be scattered around in the event of image transfer from the image carrier to a recording medium or would spread during fixation. However, the conventional developing process is apt to bring about a so-called edge effect that increases the amount of toner to deposit on fine lines, small dots and the edges of solid images. It has therefore been difficult to faithfully develop latent images representative of thin lines and small dots for thereby forming uniform images.
The edge effect that obstructs faithful reproduction occurs without regard to the type of the developer as well. The edge effect is particular serious with the two-ingredient type developer because a gap for development between the toner carrier and the image carrier is as great as several hundred micrometers.
As stated above, the edge effect makes desirable image formation difficult without regard to the type of the developing apparatus or the type of the developer. The edge effect is particularly noticeable with the developing device using the two-ingredient type developer because the gap between the toner carrier and the image carrier is as great as several hundred micrometers.
The developing device using the one-ingredient type developer is advantageous in that only the toner can be stably charged and deposited on the toner carrier, obviating irregular development despite non-contact development. However, even this kind of developing device cannot sufficiently reduce the edge effect and therefore has the problems discussed earlier.
In. light of the above, we conducted a series of extended researches and experiments and found that the edge effect could be reduced if a preselected relation was set up between the capacity of a region adjoining the surface of the image carrier and where toner contributing to development is present and the capacity of the photoconductive layer of the image carrier.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2001-34067 and 2001-117353.
It is a first object of the present invention to provide an image forming apparatus capable of implementing low-potential development, reducing background contamination, and forming an image having target density of 0.5xc3x9710xe2x88x923 g/cm2 in terms of the amount of toner deposition, and an image forming process unit therefor.
It is a second object of the present invention to provide an image forming apparatus capable of reducing the edge effect to thereby promote faithful development of a latent image, and an image forming process therefor.
In accordance with the present invention, an image forming apparatus includes an image carrier including a photoconductive layer formed on a conductive base. A latent image forming device uniformly charges the surface of the image carrier and then scans the surface with a light beam in accordance with image data to thereby form a latent image. A developing device deposits toner on a toner carrier, which includes a conductive base, and causes the toner carrier to convey the toner to a developing position where the toner carrier faces the image carrier to thereby develop the latent image and produce a corresponding toner image. A power supply applies a bias VB for development to the conductive base of the toner carrier. An image transferring device transfers the toner image from the image carrier to a recording medium. Assume that a capacity and a resistance between the conductive base of the toner carrier and the surface of the photoconductive layer in the developing region are CD (F/cm2) and RD (xcexa9cm2)D respectively. Also, assume that the photoconductive layer has a capacity of CP (F/cm2) and a resistance, as measured in the thickness of direction, of RP (xcexa9/cm2), and that a potential for development that is a difference between the potential VL of the image portion of the image carrier and the bias VB (VLxe2x88x92VB) is 300 V or below in absolute value. Then, CD, RD, CP, and RP are selected such that the amount of charge QP to be charged into the capacity CP for a unit area within a period of time in which the surface of the image carrier moves away from the developing position. is 2.5xc3x9710xe2x88x929 C/cm2 in absolute value.
Further, in accordance with the present invention, an image forming apparatus includes an apparatus body and an image carrier including a photoconductive layer formed on a conductive base. A latent image forming device uniformly charges the surface of the image carrier and then scans the surface with a light beam in accordance with image data to thereby form a latent image. A developing device deposits toner on a toner carrier, which includes a conductive base, and causes the toner carrier to convey the toner to a developing position where the toner carrier faces the image carrier to thereby develop the latent image and produce a corresponding toner image. A power supply applies a bias VB for development to the conductive base of the toner carrier. An image transferring device transfers the toner image from the image carrier to a recording medium. A region adjoining the surface of the image carrier at the developing position and where the toner contributing to development exists has a capacity CTL for a unit area greater than the capacity CPC of the photoconductive layer for a unit area.